In recent years, for substantially reducing the consumption of fossil fuel, it has been demanded and attempted to burn the biomass fuel together with the fossil fuel, such as coal or the like, in an adequately mixed state. To this end, one approach for burning the biomass fuel (e.g., the woody biomass fuel), together with the coal in the mixed state, by using a pulverized coal fired boiler has been studied. As a result, one method has been established and employed, in which a small amount of the biomass fuel is once supplied into a coal bunker and milled therein together with the coal into a powdered material, and the so-obtained powdered material is then supplied into a furnace by air and burned in the furnace.
However, in such a method that the biomass fuel is once milled together with the coal by using a coal mill and then the so-obtained powdered material is burned in the mixed state, the efficiency of milling the coal tends to be lowered, as the ratio of the biomass fuel in the powdered material is raised. Therefore, under the present conditions, the ratio of the biomass fuel to be burned together with the pulverized coal in the mixed state is limited within a range of from approximately 2 to 3% by weight.
In order to raise such a limited ratio of the biomass fuel to be burned together with the pulverized coal in the mixed state, another approach has been proposed, in which the biomass fuel is milled by an exclusive biomass mill, and then the so-milled biomass fuel is supplied to the furnace by using a separate burner different from the burner provided for the pulverized coal, and finally burned in the furnace together with the pulverized coal in the mixed state. It is true that this method can securely avoid the lowering of the efficiency of milling the coal as described above. Thus, the ratio of the biomass fuel in the milled or powdered material to be burned in the mixed state can be considerably raised, without unduly lowering the efficiency of milling the coal. However, if such increased biomass fuel cannot be adequately subjected to the suspension firing in the furnace, the efficiency of burning the fuel will be in turn degraded. Namely, in order to achieve adequate suspension firing of the biomass fuel in the furnace, the particle size of such biomass fuel should be controlled within the range of the aforementioned limit of particle size (e.g., for the wood-based or woody biomass fuel, within the range of from approximately 3 to 5 mm). However, in the case in which a considerably great amount of the biomass fuel is milled into the particles of such a relatively small particle size, i.e., within the range of from 3 to 5 mm, very great milling power should be required, leading to so great energy loss, thus rather getting out of the primary purpose in utilizing the biomass fuel.
FIG. 8 is a graph showing distribution of the particle size of the woody biomass fuel once milled by the exclusive biomass mill. In this drawing, the milled particle size less than 5 mm (<5 mm) and 5 mm milled particle size are estimated, respectively based on the particle-size distribution actually measured for the milled particle size less than 3 mm (<3 mm) of the biomass fuel. FIG. 9 is a graph showing a relationship between the average milled particle size d50 (50%-by-weight particle size) and the power unit (i.e., the unit of power required for milling the biomass fuel) (kwh/t), as described in a report opened to the public (by NEDO). As is seen from FIG. 9, the power unit plotted corresponding to the 5 mm milled particle size is lower, by about one unit or digit, than the power unit plotted corresponding to the milled particle size less than 3 mm.
Therefore, if the allowable range of the particle size of the biomass fuel milled by the exclusive biomass mill may be set equal to or greater than 5 mm, the power required for milling such biomass fuel can be significantly reduced.
For instance, in order to reduce the power required for milling the biomass fuel, JP2005-291531A (Patent Document 1) describes one technique for milling the biomass fuel into the milled particle size equal to or less than 5 mm by using the exclusive biomass mill, and then burning such milled biomass fuel together with the pulverized coal in the mixed state (hereinafter, this technique will be referred to as “the first related art.” Specifically, this technique is configured as shown in FIG. 4, wherein pulverized coal burners 4 and biomass-burners 5 are respectively provided in the same levels on a plurality of stages. Further, as shown in FIG. 4, the coal supplied from the coal bunker 11 is milled by the coal mill 6, and then fed to the furnace 1 by each pulverized coal burner 4. Meanwhile, the biomass fuel 16 is once supplied to a biomass bunker 12, and milled by the biomass mill 13, and then fed to the furnace 1 by each biomass burner 5. Thereafter, the pulverized coal and milled biomass fuel are burned together, with combustion air supplied from a wind box 3, then blown upward, and further burned in an upper region of the furnace with the combustion air supplied from an air injection port 2. In this case, the biomass fuel of a relatively small particle size will be burned while being suspended in the furnace, and then flowed out from the furnace together with exhaust gas. Meanwhile, the biomass fuel of a relatively large particle size will fall down toward bottom of the furnace (or furnace bottom) against the flow of the combustion gas while being burned.
Ideally, even in the case in which the particle size of the biomass fuel is relatively large, such biomass fuel can be completely burned and changed into ashes before the fuel reaches the furnace bottom. Actually, however, the particle size of the biomass fuel that can allow the fuel to be completely burned and changed into the ashes is limited within the range of from 3 to 5 mm. Meanwhile, in the case of burning the particles of the biomass fuel having the particle size exceeding this range, some volatile and/or moisture components of such large-sized particles can be released therefrom, as well as some carbon-based components thereof can be partly burned. However, such large-sized particles will remain unburned at a considerably high ratio or proportion, and thus fall down onto a clinker processing unit 17 provided below the furnace bottom.
Generally, in the case of milling the biomass fuel into the particles having the milled particle size equal to or less than 5 mm (i.e., 90% by weight or more of the particles having the particle size less than 5 mm, and the remaining 10% of the particles having the particle size equal to or greater than 5 mm), the power required for milling the biomass fuel tends to be increased, exponentially, with decrease of the milled particle size of the biomass fuel. Therefore, if the allowable range of the particle size of the biomass fuel milled by the exclusive biomass mill may be set greater than 5 mm (i.e., the maximum particle size is equal to or greater than 5 mm and 90% by weight of less of the particles have the particles size less than 5 mm), the power required for milling such biomass fuel can be significantly reduced. The above first related art is based on such experimental results and findings as described above, thus attempting to utilize the biomass fuel having the 5 mm milled particle size. However, as is also described above, such medium particles of the biomass fuel (i.e., the particles having the 5 mm particle size) tends to fall down toward the furnace bottom and reach the clinker processing unit 17, while remaining unburned (i.e., unchanged into the ashes). In addition, such medium particles will be cooled on the clinker processing unit 17, while still remaining unburned, and then changed into a carbonized material. Thus, for solving this problem, the first related art is also intended to collect or recover such a carbonized material by a wet separation process (such as by separating and floating the carbonized material with water from the clinker processing unit 17). Namely, such a carbonized material can be collected or recovered to be supplied again to the coal bunker 11 and milled by the coal mill 6, and then supplied and burned in the furnace. According to this method, even in the case of using the biomass fuel milled into the 5 mm particle size, such biomass fuel can be well burned together with the pulverized coal in the mixed state without degrading the efficiency of milling the coal by using the coal mill.
Further, in the above first related art, as shown in FIG. 4, a wet separation unit 14 is provided to the wet clinker processing unit, wherein this wet separation unit 14 is connected with a carbonized-material bunker 15 by means of a carbonized-material transport unit 18 which is provided between the wet separation unit 14 and the carbonized-material bunker 15.
Namely, according to this first related art, the unburned biomass fuel (or carbonized material) having fallen down on the clinker processing unit 17 can be subjected to a wet process, and then separated and collected from the clinker processing unit 17 by the wet separation unit 14. Thereafter, the so-collected carbonized material can be transported to the carbonized-material bunker 15 by the carbonized-material transport unit 18, and then supplied to the coal bunker 11 from the carbonized-material bunker 15. Thereafter, the carbonized material supplied to the coal bunker 11 can be milled together with the pulverized coal into the powdered material by the coal mill 6, and then burned by each pulverized coal burner 4.
Basically, the first related art features cooling the carbonized unburned biomass fuel by the wet clinker processing unit and then collecting such cooled unburned biomass fuel (or carbonized material) by the wet separation unit 14. However, this related art also implies use of a dry clinker processing unit.
In addition, according to the above first related art, the biomass fuel (or carbonized material) that has been cooled and then collected via the wet clinker processing unit is well carbonized and includes the medium particles b as shown in FIG. 5. Therefore, such biomass fuel or carbonized material is likely to be milled, showing very low resistance against the milling performed by the coal mill 6.
However, if the biomass fuel contains a relatively large amount of coarse particles (i.e., coarse particles B also shown in FIG. 5) having the particle size greater than 5 mm, a correspondingly large amount of woody cores may tend to remain in the unburned or carbonized material when the material is recovered from the clinker processing unit 17. Therefore, if such carbonized material containing such a great amount of the woody cores is supplied to the coal mill, the efficiency of milling the coal may tend to be rather degraded.
From such findings, in the above first related art, the particle size of the biomass fuel to be burned together with the pulverized coal in the mixed state is limited within the range that can allow the particles of the biomass fuel to adequately fall down toward the furnace bottom as well as allow such particles to be completely carbonized.
Namely, if the particle size of the biomass fuel is unduly great (e.g., greater than 7 mm), such coarse particles will fall down in a considerably great amount onto the clinker processing unit 17 with an unduly large amount of the woody cores remaining therein. Accordingly, the milled particle size of the biomass fuel should be controlled not to be so great. In addition, as the particle size is considerably increased, the speed of the particles falling downward in the furnace will be raised so much, thus rather reducing a period of time during which such particles can be subjected to the suspension firing in the furnace, leading to production of an unduly large amount of the unburned material.
It should be noted that the above Patent Document 1 is silent about any specific structure of the dry clinker processing unit. In other words, this reference suggests or teaches nothing about the structure of such a dry clinker processing unit, in particular, about the mechanism or method of cooling the unburned biomass fuel or carbonized material. Meanwhile, the dry clinker processing unit itself has publicly known, so far, as a “clinker processing unit”, and one example of such a dry clinker processing unit is described in JP7-56375B (Patent Document 4). FIG. 7 schematically shows such a publicly known clinker processing unit. Namely, as shown in FIG. 7, this dry clinker processing unit includes a conveyor belt 23 provided below the furnace bottom and made of a highly heat-resistant metal. With this configuration, the ashes can fall down onto the conveyor belt 23, while being guided through a transition hopper 20 provided between the furnace 1 and the conveyor belt 23. In this case, the conveyor belt 23 is driven by one or more drive wheels or rollers 24.
Further, this dry clinker processing unit includes a casing 22 having a hermetically sealed structure. Additionally, several of cooling-air intake holes 31 are provided on one side of the conveyor belt 23, such that cooling air can be supplied into the clinker processing unit.
With the provision of this dry clinker processing unit, the ashes having fallen on the bottom of the furnace 1 can be received or caught by the conveyor belt 23, and transferred slowly (e.g., at 5 mm per second) toward a clinker collecting device 41. During this transfer operation, the ashes can be slowly cooled by the air for about one hour (i.e., the time required for each transfer operation of the conveyor belt 23). Then, the so-transferred-and-cooled ashes are finally discharged from the clinker processing unit and collected by the clinker collecting device 41. Therefore, according to this dry clinker processing unit 21, the cooling air is supplied into the body or casing of the clinker processing unit 21, such that the ashes having fallen down on the conveyor belt 23 can be gradually cooled during the transfer thereof through the body of the clinker processing unit 21. Thereafter, the so-cooled ashes can be discharged to the outside from the clinker processing unit 21. Meanwhile, the cooling air supplied into the body of the clinker processing unit 21 is heated by the burned ashes up to a high temperature around the furnace bottom, and then drawn upward into the furnace 1 to be confluent with the combustion gas present in the furnace 1.
However, in the case in which this dry clinker processing unit is applied to the pulverized coal fired boiler, the amount of the cooling air supplied into the dry clinker processing unit should be limited to approximately 2% relative to the amount of the combustion air directly supplied to the furnace. Further, in this case, the ashes can be cooled to approximately 100° C., at or around the furnace bottom, for the period of time (i.e., about one hour) during which the ashes are transferred through the body (see FIG. 7) of the clinker processing unit to the discharge point thereof.
By the way, another related art, which includes the biomass burners respectively arranged above the pulverized coal burners, in order to suspend the biomass particles on an ascending flow or current of the air and other gases and thus lengthen the time for which such biomass fuel can be burned in the furnace, has also been known (see FIG. 6). Namely, in this boiler system, the biomass burners 5 are respectively located in an upper portion of the furnace 1, while the pulverized coal burners are respectively located in a lower portion of the furnace 1. In other words, a combustion region F1 for the pulverized coal is provided in the lower portion of the furnace 1, while another combustion region F2 for the biomass fuel is provided in the upper portion of the furnace 1. With this configuration, the speed of the biomass fuel falling downward in the furnace 1 can be successfully lowered, by utilizing the ascending flow of the air and other gases created by the flame of the respective pulverized coal burners 4, thereby lengthening the period of time during which the biomass fuel can be suspended in the furnace 1. This related art is disclosed in both of JP2007-101135A (Patent Document 2) and JP2005-241108A (Patent Document 3), and will be referred to as “the second related art.” In fact, with the configuration according to the second related art including the biomass burners respectively located above the pulverized coal burners 4, the period of time during which the biomass fuel can be burned in the upper combustion region in the furnace 1 can be somewhat lengthened, as compared with the first related art. Therefore, this second related art can allow the biomass fuel having the relatively large milled particle size to be used for the burning process in the boiler. However, with the increase of the milled particle size of the biomass fuel, the ratio or proportion of the coarse particles B having the particle size equal to or greater than 5 mm is raised. Thus, even if the period of time during which the biomass material can be burned in the furnace can be lengthened to some extent by arranging the respective biomass burners 5 above the respective pulverized coal burners 4, the problem that the woody cores tend to remain unburned in a greater amount cannot be solved.
In addition, for recovering the carbonized material of the biomass fuel and then supplying the so-recovered material to the coal bunker, the above first related art (Patent Document 1) related to the mixed-fuel firing boiler further requires the wet separation unit, transport unit, carbonized-material bunker, carbonized-material mill and the like to be respectively provided thereto. Therefore, for an existing or current coal fired boiler, considerably large-scale equipment should be added thereto in order to adequately burn the biomass fuel together with the pulverized coal in the mixed state, thus unduly increasing the cost for the equipment.
Therefore, in order to significantly raise the ratio or proportion of the biomass fuel in the mixed fuel or material to be burned in the furnace as well as to improve the efficiency and effect of utilizing such biomass fuel, with the cost required for the operation and equipment of the boiler being substantially reduced, it is necessary to provide a significantly improved mechanism or method of burning the biomass fuel together with the pulverized coal in the mixed state in the pulverized coal fired boiler with highly enhanced combustion efficiency and effect. Namely, even in the case in which the biomass fuel of the relatively large particle size is supplied to the furnace of the boiler, this mechanism or method needs to significantly raise the ratio or proportion of such biomass fuel to be burned together with the pulverized coal in the mixed state, as well as can burn such biomass fuel completely into the ashes. Besides, this mechanism or method needs to be achieved and implemented without requiring unduly large-scale additional equipment.